The present invention relates to a method of purifying an acid leaching solution which has been obtained by hydrometallurgically working-up material that contains valuable metals, and that in addition to said valuable metals also includes Fe3+ and Fe2+ and possibly also arsenic in solution. Although the method can be used generally for acid leaching solutions of this nature regardless of the choice of hydrometallurgical process applied and the type of starting material, it is particularly suitable for purifying acid leaching solutions that result from bioleaching processes, i.e. processes in which bacteria are used to accelerate the leaching process.
Bioleaching is described generally in our earlier Patent Publications U.S. Pat. No. 5,397,380 and SE-A-9901613-1, and, for instance, also in WO 9216667 which deals generally with the oxidation of metal sulphide material with thermo-tolerant bacteria and also in WO 94/28184 which relates to bioleaching of zinc concentrate.
It is necessary to cleanse leaching solutions of the kind described in the introduction of their iron and possible arsenic content, in order to be able to win their valuable metal contents economically, for instance their copper, zinc, nickel, cobalt and precious metal contents by electrowinning or by some other suitable process. In the known methods applied to this end, such acid solutions, which often have a pH of about 1, are neutralised with lime or some like pH-elevating agent so as to precipitate the iron, wherewith any arsenic present will also be precipitated at the same time. Total precipitation of the iron (III) content is obtained at pH 3.0-3.5. One such known method is described in AU-A-11201/92, for instance.
A relatively large percentage of the iron in a bioleaching solution is present in divalent form, Fe2+, and it is necessary to convert all iron to a trivalent form, i.e. Fe3+, by oxidation in order to be able to separate the iron from remaining metals (valuable metals) by precipitation. This is normally effected at a pH greater than 3 by injecting air into the system so as to obtain a sufficiently rapid kinetic, in other words a high oxidation rate. However, when precipitating iron at such high pH values some precipitation of other metals will take place, caused by inclusions in the resultant voluminous iron hydroxide precipitate among other things, and consequently valuable metals will be lost in the precipitate to an extent which is economically significant with respect to most valuable metals.
Proposals have been made as to how the loss of valuable metals caused by this co-precipitation can be avoided or at least reduced. There is described in an article by A. P. Brigos et al (Int. Biohydrometallurgy Symposium IBS97, Sydney, 1997) a bacteria leaching process undertaken in Uganda to recover cobalt from roasted pyrites, in which the iron was precipitated incompletely in order to avoid cobalt losses. This process, however, results in problems in the following cobalt recovery stage.
Another article by M. L. Steemson et al in the same publication (IBS97), there is described a bioleaching process for treating zinc concentrate, in which co-precipitation of zinc with the precipitation of iron is particularly upheld as a serious problem that necessitates re-dissolving the resultant iron precipitate and re-filtration in order to obtain a solution of the purity required for the subsequent zinc winning process using solvent extraction/electrowinning.